Dispersion compensating devices are essential building blocks of high-speed optical communications systems. Important requirements include low loss and the ability to compensate the dispersion at every wavelength of a wavelength-division-multiplexed (WDM) lightwave system. In addition to static dispersion compensation, high-speed optical communications systems also require tunable dispersion-compensators (TDC). They facilitate offsetting variations in dispersion in a fiber optic transmission line. The variations may be due to environmental changes (varying stress or temperature of the transmission fiber and components), power fluctuations leading to varying nonlinear phase shifts, or dynamic reconfigurations of networks that alter the path lengths of various WDM channels. See B. J. Eggleton et. al, J. Lightwave Tech., vol. 18, p. 1419 (2000). Alternatively, the variations may arise from statistical fluctuations of dispersion in the transmission fiber, as well as statistical variations in the length of transmission fiber between adjacent amplifier huts.
To date, several tunable or adjustable dispersion compensators have been proposed and demonstrated. Chirped fiber-Bragg-gratings (FBG) have been used extensively to tune the dispersion of lightwave signals. For example, a FBG with linear or nonlinear chirp can be tuned by a heating element or a latchable magnetic strain, to vary the dispersion of the device. See U.S. Pat. Nos. 6,148,127 and 6,330,383. Dispersion tuning ranges of ˜2000 ps/nm over bandwidths of 1 to 1.5 nm have been demonstrated by this technique. The limited bandwidth of such tunable devices restricts its use to single channel applications. Using this device in a WDM system would entail de-multiplexing the signal into individual wavelength channels, and using a distinct FBG-based TDC for each channel, making it very costly. Alternatives to single channel FBG-TDCs include sampled FBGs that can compensate the dispersion for three or four channels simultaneously. While this reduces the number of devices needed in a WDM system by a factor of three to four, it still remains a costly means to implement tunable dispersion management. Moreover, all TDCs that utilise FBGs suffer from group-delay (GD) ripple impairments that lead to bit-error-rate (BER) power penalties. Further, the GD-ripple increases with bandwidth or dispersion of the device. An additional complication is that a practical FBG based TDC would entail fabricating FBGs on meter-lengths of fibers for dispersion-compensation over an entire communication band. Fabrication and tuning methods for such long gratings would appear impractical.
An alternate technique is to use the variable phase response of optical filters to tune dispersion. Planar waveguide-based all-pass filters have been demonstrated to provide TDC with tuning ranges of up to 500 ps/nm at 40 Gb/s. See C. K. Madsen, Proc. Optical Fiber Conf. 2002, papers No. TUT-1 and FD-9. The dispersion of virtually imaged phase array (VIPA) devices can be tuned by translating a specially designed free-space mirror, as described in U.S. Pat. No. 6,392,807. This device has been demonstrated to provide +/−800 ps/nm tuning range. See Shirasaki, et al., Proc. European Conf. Optical Comm.—2000, PD-2,3. Both these technologies, as well as several others that utilize the phase response of optical filters, are periodic with respect to wavelength, and can thus provide simultaneous compensation to all channels as long as they are designed to have a periodicity coincident with the WDM channels. However, all such devices suffer from a wavelength dependent response within each “pass-band”. Hence, these devices may not be suitable for high bit-rate applications because of the trade-off between dispersion and bandwidth. They also suffer from GD ripple impairments like FBG-based TDCs. Finally, phase-response based devices require coupling light in and out of the transmission fiber, which makes them lossy.
Thus, there exists the need for a device that can offer tunable or adjustable dispersion with an optical performance similar to that of dispersion-compensating fibers (DCF) or higher-order-mode dispersion-compensating modules (HOM-DCM) commonly used for static dispersion compensation. The desirable features would be low loss, low multi-path interference, negligible GD ripple, and most importantly, a response that is continuous in wavelength.